the roman glass industry
This week we took glass production as one illustration of
Roman trade and technology.
Glass was made in the Nile Valley and Fertile Crescent by 1800 BC. Before this, in the same geographical area, a kind of ‘glass clay’ that we call ‘faїence’ had been made and used for thousands of years. Glass clay – faїence – is made of a mixture of sandy clay, copper salts and natron or sodium carbonate. The mix is worked like clay and can be shaped and moulded cold. It’s air-dried then baked at a relatively low temperature. The copper salts turn the fired piece varying shades of turquoise-blue. Faїence objects were mass-produced in the same way that pottery was, and they continued to be made in the Roman period. But proper glass needs higher temperatures to make it, and it probably arose on the edges of both pottery firing and metal working as both of these also require high temperature kilns. Glass is made of silica, lime and flux. Silica = sand, and handily the necessary lime was also in the sand used initially in places like the Nile Valley. The flux used was natron – sodium carbonate from the Wadi Natrun alkali lakes in the western desert. This was also the source of the main chemical used in the mummification process. During ancient times natron was used as flux, whereas in later times crushed flint and lead silicates were used. The presence of natron flux is therefore one of the chemical indicators that a piece of glass is from ancient or classical times and not post-1000 AD. Natron and sand are heavy low-value commodities, and so glass production (as opposed to glass working) remained closer to these sources. It’s quite clear from archaeological evidence that enormous amounts of colured glass were being produced around the eastern end of the Mediterranean, and that these batches were broken up and sold on as part of the sea-based trade of the time. Once made, glass re-melts at a lower temperature and can then be reworked. So it makes better economic sense to sell the glass a bit like metal ingots are sold, and leave the fancy design work to others. Glass was acquired by northern Europeans – Celtic and Germanic tribes – who quickly learned how to made ‘lamp-glass’ beads ie small quantities of crushed glass were kiln-fired then trailed around a metal rod. This is pretty much the same technique used by glass novelty makers today. This trade in glass ‘cake’ for small-scale use was lively and widespread, and as the Romans expanded their territories, it continued pretty much as before, making it often hard to tell if a particular bead dates from the Roman or pre-Roman age. Initially, up to the turn of the Common Era, glassworkers treated molten glass like clay, in that objects were made by trailing glass around a clay core and leaving to cool. Glass was usually opaque, and brightly coloured. Colours in glass were and are still produced by adding copper, iron, and tin compounds to the mix in the kiln. Glass was also occasionally used like semi-precious stones, and was carefully cut into designs like gemstone cameo jewels. The Portland Vase is an excellent, rare example of a large glass object made using the ‘cameo cutting’ technique. Around the turn of the Common Era the technique of blowing glass was discovered, again probably in the Greek-speaking world of the eastern Mediterranean. The Portland Vase was also produced initially by glass blowing, the basic vessel shape being comprised of a dark blue ‘bubble dipped into white opaque glass, cooled, and then the white glass carefully cut back to make the cameo shapes. Glass blowing made it easier to make very large numbers of vessels quickly. It could be combined with moulds, so fancy ware rather reminiscent of Victorian pressed glass could be made and also be widely afforded. At the same period, it became fashionable to have clear glass rather than opaque brightly-coloured glass. One side-effect of the expansion of the Roman Empire was populations grew – hence markets for all commodities grew. It is clear from the quantities of glass objects recovered from Roman sites that people wanted glass vessels. Glass was also acquired and recycled, rather than being disposed of in domestic tips, so was considered valuable even when broken. To a society that had shops, glass enabled retailers to display goods attractively and volatile products such as unguents, medicines and perfumes could be safely ‘bottled up’, retaining their top quality for longer. Innumerable small long-necked glass bottles come from Roman period burial sites. In the 19th century it was believed that they were used for gathering mourners’ tears (!), but they appear to be perfume bottles left either as gifts to the departed or containing a ‘shot’ of expensive perfume used as a libation during the funeral. Larger glass storage jars have survived intact where they were re-used as cremation urns. It seems alien to us, but the Romans seemed comfortable with re-using these high-end storage jars in this way, and it strikes me that the glass vessels may have facilitated the Roman practice of re-visiting the deceased to offer prayers and libations. At the luxury end of the market, beautiful clear glass cremation urns were made, probably at one Italian workshop, complete with a glass lid with integral funnel, allowing any amount of libations to be added without disturbing the ashes within. These are few and far between compared with the vast quantity of everyday small Roman glass objects that can be seen in museums and are sold via the trade in antiquities. So where were the glassworkers based? The simple answer is, it depends on the scale of the work. Beadmakers and enamellers might be unidentifiable archaeologically, as they would use small amounts of glass and perhaps be operating within another workshop such as a bronze-smiths. A small glass kiln was excavated recently in London. It’s known that major glassworking took place near cities on the Rhine such as Cologne. It’s impossible to identify where a glass vessel was made from its chemical components, as this would simply tell us where the glass itself was first fused ie somewhere in the eastern Mediterranean. So this comes out as an unsatisfying ‘we can’t be 100% sure’. The Romans seem to have been the pioneers of window-glass. Fragments thought to be window-glass have been recovered from a few bath-house sites, where it seems thick glass squares were fixed together to make simple skylights in the hottest rooms. This glass was translucent and heavy and isn’t found even in luxury palace sites, other than in hot rooms of the bath-house. |